Nitrogen in ryegrass (Lolium perenne L.) : seed yield and indicators of nitrogen status

Abstract

Nitrogen dominates plant nutrition and in ryegrass seed crops the availability of nitrogen is a major determinant of seed yield, affecting all yield components. Nitrogen fertiliser is readily used but rarely efficiently. Numerous trials have been carried out to determine the optimum rate and time of nitrogen inputs but these are associated with numerous problems. Hence there is a move away from the traditional field experiments to more widely applicable techniques such as plant based indicators of nitrogen requirements and hence yield ego herbage nitrogen concentration, sap nitrate and chlorophyll concentration (as indicated by the chlorophyll meter). It is important, for both economic and environmental reasons, that plant nitrogen status be measured to enable predictions of fertiliser requirements over a wide range of field conditions. A field trial was established to investigate the effect of different rates and timing of nitrogen fertiliser on seed yield of perennial ryegrass. There were 16 treatments arranged in a Randomised Complete Block design. Several plant based indicators were also investigated as indicators of nitrogen status. These were herbage nitrogen concentration, sap nitrate (Merckoquant nitrate strips and Nitracheck machine) and chlorophyll concentration (Minolta chlorophyll meter SP AD-502 and DMSO extraction). Sap nitrate and the chlorophyll meter were tested in a separate experiment carried out in the field and glasshouse. A significant response of seed yield to nitrogen was observed in the field trial, however, only the control was significantly different to the other treatments (ON = 450 kg/ha; N treatments: average = 1874 kg/ha, range = 1560 to 2291 kg/ha). This was attributed to a drought event at stem elongation due to incorrect irrigation management. There was compensation among yield components, particularly head numbers (1066 to 1999 heads/m²) and floret site utilisation (12.31 to 33.19%) to achieve similar seed yield results for a wide range of nitrogen treatments. A significant linear relationship was observed between herbage nitrogen and yield (R² = 0.58); this showed significant departure from previous work due to the compromise in soil moisture status. Sap nitrate could not be used as an indicator of herbage nitrogen concentration as although it differentiated between those treatments which were deficient and those which were sufficient (i.e. showed luxury consumption of nitrogen) there was no indication of how deficient or 'in surplus' the plants were. Thus sap nitrate has no potential as an indicator of fertiliser requirements in ryegrass. Use of the Nitracheck machine was no improvement over Merckoquant nitrate strips. An increase in SPAD reading was associated with an increase in herbage nitrogen concentration; this was a significant, linear relationship (R² = 0.74 and 0.78 for field and glasshouse experiment respectively). An increase in SPAD was also associated with an increase in total chlorophyll extracted with DMSO (R² = 0.71, for chlorophyll expressed on a dry weight basis). The chlorophyll meter has shown promise as a rapid, reliable indicator of nitrogen status in perennial ryegrass. Further work is necessary to establish the relationship between chlorophyll concentration, as measured by the chlorophyll meter, and seed yield. Other factors requiring investigation, which would enable a best management practice for perennial ryegrass seed crops to be identified, are discussed.